An inverter comprising a H-shaped inverter bridge made of semiconductor switches and connected between two input lines and a controller controlling the semiconductor switches in operation of the inverter is, for example, known from US Patent Application Publication US 2005/0286281 A1 (corresponding to German Patent DE 10 2004 030 912 B3). Here, the semiconductor switches are MOSFETs, i.e. metal oxide semiconductor field effect transistors. Such field effect transistors are of the normally off-type, i.e. without applying control voltages to their gates they are not conductive. This is an advantage in so far as the entire inverter bridge is not conductive as long as no control voltages are present yet, and as soon as any control voltages are no longer present. Thus, the normally not conductive inverter bridge avoids a short circuit both between the input lines and between AC output lines at any time at which the controller is not available.
In the particular inverter known from US 2005/0286281 A1, an additional semiconductor switch which also is a MOSFET is provided in one of the input lines. This additional semiconductor switch in the one input line and the semiconductor switches of the inverter bridge which are connected to the other input line are operated by the controller at a higher frequency, whereas the two other semiconductor switches of the inverter bridge which are connected to the one input line are operated by the controller at a lower frequency. This lower frequency corresponds to the frequency of the AC voltage output by the inverter, whereas the higher frequency is used for shaping the output AC current by means of pulse width modulation. In the inverter known from US 2005/0286281 A1 all semiconductor switches being MOSFETs are provided with anti-parallel diodes.
A press release No. 02/08 by Fraunhofer-Institut für Solare Energiesysteme ISE dated Jan. 15, 2008 (http://www.ise.fhg.de/presse-und-medien/presseinformationen-pdf/0208_ISE_Pl_d_Rekord_Wechselrichterwirkungsgrad.pdf) reports a record efficiency of an inverter. MOSFETs on the basis of the semiconductor material silicone carbide (SiC) are reported to have been used in this inverter. SiC semiconductor parts are known both to have good electric properties like low conducting and switching losses and to be suitable for use in a much greater temperature range than common silicon semiconductor parts, i.e. up to 600° C. MOSFETs on the basis of SiC, however, are not generally available at present. Thus, they can not be widely used in inverters to raise the temperature limit posed by common silicon semiconductor parts.
Junction-gate field effect transistors (JFETs) on a SiC basis, however, are available on acceptable terms. JFETs, however, are principally semiconductor switches of the normally on-type, i.e. they are conductive without control voltages being applied to their gates. To compensate for this disadvantage, a so-called cascode is known in which a JFET is combined with a MOSFET and in which an input voltage is branched to the gate of the JFET and, via the MOSFET, to the source of the JFET. In the cascode, the JFET is indirectly switched or operated by controlling the gate of the MOSFET. It is an advantage of the cascode that it makes use of the high blocking capability of the SiC JFET and that the MOSFET only needs to have a small blocking voltage resistance. However, the power current through the cascade also flows through the MOSFET which essentially influences the switching properties of the entire cascade so that a cascode bears nearly all disadvantages of a MOSFET as compared to a JFET.
In an inverter known from EP 2 006 991 A1, normally on semiconductor switches of an inverter bridge are each provided in a cascade. As explained above, the advantages of a JFET may only be exploited to a limited extent in a cascade. Particularly, no use can be made of the high temperature resistance of a JFET on a SiC basis due to the low temperature resistance of the MOSFET arranged in close proximity to the JFET in the cascade. Further, the total number of semiconductor switches is twice as high as normally required when using cascodes as switches of an inverter bridge, each cascode comprising one JFET and one MOSFET.
An electronic switching device having at least two semiconductor switching elements, one of which is a normally on semiconductor switch, the other of which is a normally off semiconductor switch and which are connected together in series, is known from DE 10 2006 029 928 B3. Here, the switching device has two load contacts leading to the series connection and two control contacts for operating the two semiconductor switches by a controller. The controller may thus operate the normally on semiconductor switch by temporarily applying a control voltage to its gate whereas it continuously keeps the normally off semiconductor switch conductive by permanently applying a control voltage to its gate. Thus, it is possible to make use of the advantageous switching properties of the normally on semiconductor switch. Further, this known switching device comprises a decoupling means which only decouples both semiconductor switches, when the controller in fact controls the normally on semiconductor switch. Prior to and after that, both semiconductor switches are coupled in the same way as in a cascade.
There still is a need for a converter which, to a further extent, makes use of the advantages of semiconductor switches on SiC basis which are available at low cost.